Sagittarius B2

Sagittarius B2 (Sgr B2) is a giant molecular cloud of gas and dust that is located about 120 parsecs from the center of the Milky Way. This complex is the largest molecular cloud in the vicinity of the core and one of the largest in the galaxy, spanning a region about 45 parsecs across.cite news last=Chown | first=Marcus | date=November 27, 1999 title=Star attraction | publisher=New Scientist url=http://www.newscientist.com/article/mg16422144.900-star-attraction.html accessdate=2007-10-29] The total mass of Sgr B2 is about 3 million times the mass of the Sun. [cite book first=P. M. | last=Solomon editors=Giancarlo Setti and Giovanni G. Fazio year=1978 title=Physics of Molecular Clouds from Millimeter Wave Length Observations work=Infrared Astronomy | publisher=Springer location=New York | id=ISBN 9027708711 ] The mean hydrogen density within the cloud is 3000 atoms per cm3, which is about 20&ndash;40 times denser than a typical molecular cloud. [cite journal last=Goldsmith | first=Paul F. coauthors=Lis, Dariusz C.; Hills, Richard; Lasenby, Joan title=High angular resolution submillimeter observations of Sagittarius B2 journal=Astrophysical Journal year=1990 | volume=350 | pages=186–194 url=http://adsabs.harvard.edu/cgi-bin/bib_query?1990ApJ...350..186G accessdate=2007-10-31 doi=10.1086/168372 ]

The internal structure of this cloud is complex, with varying densities and temperatures. The cloud is divided into three main cores, designated north (N), middle or main (M) and south (S) respectively. Thus Sgr B2(N) represents the north core. The sites Sgr B2(M) and Sgr B2(N) are sites of massive star formation. The first 10 H II regions discovered were designated A through J. [cite journal last=Lis first=Dariusz C. coauthors=Goldsmith, Paul F. title=Modeling of the continuum and molecular line emission from the Sagittarius B2 molecular cloud journal=Astrophysical Journal, Part 1 year=1990 volume=356 pages=195–210 url=http://adsabs.harvard.edu/cgi-bin/bib_query?1990ApJ...356..195L accessdate=2007-10-25 doi=10.1086/168830] H II regions A-G, I and J lie within Sgr B2(M), while region K is in Sgr B2(N) and region H is in Sgr B2(S). [cite journal last=Takagi | first=Shin-ichiro coauthors=Murakami, Hiroshi; Koyama, Katsuji title=X-Ray Sources and Star Formation Activity in the Sagittarius B2 Cloud Observed with Chandra journal=The Astrophysical Journal year=2002 | volume=573 | pages=275–282 url=http://adsabs.harvard.edu/abs/2002astro.ph..3035T accessdate=2007-10-31 doi=10.1086/340499 ] The 5-parsec-wide core of the cloud is a star-forming region that is emitting about 10 million times the luminosity of the Sun. [cite book first=Ramon D. | last=Wolstencroft coauthors=William Butler Burton | year=1988 title=Millimetre and Submillimetre Astronomy publisher=Springer | id=ISBN 9027727635 ]

Temperatures in the cloud vary from 300 K in dense star-forming regions to 40 K in the surrounding envelope. [cite conference first =P. last =de Vicente coauthor=Martin-Pintado, J.; Wilson, T. L. title =A Hot Ring in the SGR B2 Molecular Cloud booktitle =Proceedings Astronomical Society of the Pacific Conference Series pages =64&ndash;67 publisher =Astronomical Society of the Pacific. date =March 10-15, 1996 location =La Serena, Chile url =http://adsabs.harvard.edu/abs/1996ASPC..102...64D accessdate = 2007-10-29 ] Because the average temperature and pressure in Sgr B2 are low, chemistry based on the direct interaction of atoms is exceedingly slow. However, the Sgr B2 complex contains cold dust grains consisting of a silicon core surrounded by a mantle of water ice and various carbon compounds. The surfaces of these grains allow chemical reactions to occur by accreting molecules that can then interact with neighboring compounds. The resulting compounds can thenevaporate from the surface and join the molecular cloud.

The molecular components of this cloud can be readily observed in the 102&ndash;103 m range of wavelengths. About half of all the known interstellar molecules were first found near Sgr B2, and nearly every other currently known molecule has since been detected in this feature. [cite journal author=S. E. Cummins, R. A. Linke, P. Thaddeus title=A survey of the millimeter-wave spectrum of Sagittarius B2 journal=Astrophysical Journal Supplement Series | year=1986 | volume=60 pages=819–878 | url=http://adsabs.harvard.edu/abs/1986ApJS...60..819C accessdate=2007-02-06 doi=10.1086/191102 ]

The European Space Agency's gamma-ray observatory INTEGRAL has observed gamma rays interacting with Sgr B2, causing x-ray emission from the molecular cloud. This energy was emitted about 350 years before by the supermassive black hole (SMBH) at the galaxy's core. The total energyfrom this outburst is an estimated million times stronger than the current output from the SMBH. [cite news author=Staff | date=January 28, 2005 title=Integral rolls back history of Milky Way's super-massive black hole publisher=Hubble News Desk url=http://www.esa.int/SPECIALS/Integral/SEMSKPO3E4E_0.html accessdate=2007-10-31 ]

Sagittarius — Sag it*ta ri*us, n. [L., literally, an archer, fr. sagittarius belonging to an arrow, fr. sagitta an arrow.] (Astron.) (a) The ninth of the twelve signs of the zodiac, which the sun enters about November 22, marked thus [[sagittarius]] in… … The Collaborative International Dictionary of English

Sagittarius — constellation, late 14c., from L., lit. archer, properly pertaining to arrows, from sagitta arrow, which probably is from a pre Latin Mediterranean language. Meaning person born under Sagittarius (properly Sagittarian) is attested from 1940 … Etymology dictionary

Sagittarius — [saj΄ə ter′ē əs] n. [ME &LT; L sagittarius, archer &LT; sagitta, arrow] 1. a large S constellation in the brightest part of the Milky Way, beyond which lies the center of our galaxy; the Archer 2. the ninth sign of the zodiac, entered by the sun… … English World dictionary